Suppose a new survey lists a catch-basin rim at 101.2 feet and an older utility record lists the same basin’s pipe invert—the elevation at the inside bottom of the pipe—at 98.9 feet. Subtracting gives an apparent 2.3 feet, but that number is meaningful only if both elevations use compatible units and the same known vertical reference. A vertical datum is the reference from which elevations are measured; if the record does not identify its datum, the designer cannot safely compare the two values.
Know what the difference would mean
If both values accurately describe the same structure and use a reconciled elevation reference, their difference represents the vertical separation between that rim and pipe invert. It does not by itself establish the fall available from a proposed drain to the pipe. That design question also depends on the proposed connection elevation, route and other site conditions. A plausible subtraction is therefore neither proof of a usable connection nor a substitute for the missing design information.
Find the reference behind each elevation
The survey should identify its vertical datum, units and the benchmarks used to establish elevations. The utility record may have its own reference, an older local basis or no stated basis at all. For covered Bellevue land-use submittals, the City’s survey requirements specify NAVD 1988 and City vertical-control benchmarks. That Bellevue requirement does not tell the designer what reference an older utility record used.
If both references are documented but differ, a qualified surveyor and engineer should review the appropriate transformation before the values are combined. An unexplained numerical offset is not a correction. Keep the original record value and its source traceable beside any approved conversion.
When the record omits its datum
Identify the invert as record-derived and note that its elevation reference is unknown. Obtain the missing reference or a verified measurement before using it to set a connection grade. The surveyor and engineer can assign that verification before fieldwork; Terrane’s guide to starting survey and engineering together describes the broader coordination. The 2.3-foot figure should stay an apparent difference until its sources can actually be compared.
